Redox diagrams

Redox diagrams are used to express the stability of dissolved species and minerals. An example diagram is presented in Fig. 2.4, where the redox potentials of various types of aqueous systems are shown as a function of pH. It can be seen that at acidic pH, a mine water system has a very high oxidation potential (Eh > 500 mv). In... 

Figure 15.20 Redox diagram for U, Np, Pu and Am in 1M HC104 at 25°C (Choppin, Liljenzin, and Rydberg, 2002).

Figure 4. Redox diagrams for Fe. (a) Log (a/ Fe2+ ) as a function of pE for Fe°, Fe2+ and Fe3+. Where the Fe° line is broken, solid Fe cannot exist at equilibrium, (b) Log a as a function of pE for Fe°, Fe2 and Fe3+. The total Fe concentration is assumed to be 0.1 M except in the range where solid Fe exists at equilibrium (13)...

Figure 5. Redox diagram giving, as functions of pE, log c (solid) or log p (broken) for the main variable species in a model system corresponding to sea water 4- air + sediments. At the lower end the ranges are indicated in which various solids would be stable (15)...

The cooperation between photosystem I and photosystem 11 creates electron flow from H2O to NADP+. The pathway of electron flow is called the Z scheme of photosynthesis because the redox diagram from P680 to P700 looks like the letter Z (Figure 19 22). 

Based on the redox diagrams in Figure 16.6, estimate the reduction potentials for the one-electron step in the following series of plutonium species in acid solution Pu(IIl), Pu(TV), Pu(V), and Pu(VI). 

FIG. 12 Photocurrent transient responses associated with the heterogeneous quenching of the dimer ZnTPPS/ZnTMPyP by tetracyanoquinodimethane (TCNQ) at a water DCE interface. The redox couple FelCNlg /FelCN)/ was used as supersensitizer in the aqueous phase. The back electron transfer reaction, responsible for the photocurrent decay in the on-transient, is significantly quenched in the presence of the supersensitizer. According to the redox diagram in (b), the overall process at A 4>= —0.11 V corresponds to the reduction of TCNQ by the redox couple in the aqueous phase photocatalyzed by the porphyrin complex. Reprinted from Ref. 109 with permission from Elsevier Science. 

Develop the redox diagram (pE vs. log [CF]) for the copper system in a chloride medium. Place vertical lines in this diagram (for processes that are independent of E) in an analogous manner to the vertical lines in Figure 7-3 and 7-4. These represent the log PcuCU values. The vertical line corresponding to solid CuCl would be placed at pCl = 6.7, if it were not for the fact that Cu(I) disproportionates to Cu" and Cu° at the lower pCl value of 3.9. The horizontal lines involve redox processes in which both oxidized and reduced forms contain the same number of complexed chlorides. 

Determination of redox potentials of couples involving the ground state and the determination of by photophysical measurement allows us to construct the following redox diagram. 

In order to understand and to predict the redox behaviour of the MLCT excited state Ru(bipy)3 , it is necessary to keep in mind the redox diagram represented below. 

Re(I)(CO)3(Cl)phen (facial isomer) is luminescent in fluid solution [8] the MLCT excited state is long lived ( 0.6 s), its energy being 2.3 eV above that of the ground state. The redox diagram of Re(CO)3(Cl)phen and its MLCT excited state is represented below ... 

The diagram gives regions of existence, i.e. for a particular combination of pH and redox potential it can be predicted whether it is thennodynamically favourable for iron to be inert (stable) (region A), to actively dissolve (region B) or to fonn an oxide layer (region C). 

Ladder diagrams can also be used to evaluate equilibrium reactions in redox systems. Figure 6.9 shows a typical ladder diagram for two half-reactions in which the scale is the electrochemical potential, E. Areas of predominance are defined by the Nernst equation. Using the Fe +/Fe + half-reaction as an example, we write... 

The ladder diagram for this system is shown in Figure 11.24a. Initially the potential of the working electrode remains nearly constant at a level near the standard-state potential for the Fe UFe redox couple. As the concentration of Fe + decreases, however, the potential of the working electrode shifts toward more positive values until another oxidation reaction can provide the necessary current. Thus, in this case the potential eventually increases to a level at which the oxidation of H2O occurs. 

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

Figure 2.11 Beta sheets are usuaiiy represented simply by arrows in topology diagrams that show both the direction of each (3 strand and the way the strands are connected to each other along the polypeptide chain. Such topology diagrams are here compared with more elaborate schematic diagrams for different types of (3 sheets, (a) Four strands. Antiparallel (3 sheet in one domain of the enzyme aspartate transcarbamoylase. The structure of this enzyme has been determined to 2.8 A resolution in the laboratory of William Lipscomb, Harvard University, (b) Five strands. Parallel (3 sheet in the redox protein flavodoxin, the structure of which has been determined to 1.8 A resolution in the laboratory of Martha Ludwig, University of Michigan, (c) Eight strands. Antiparallel barrel in the electron carrier plastocyanln. This Is a closed barrel where the sheet is folded such that (3 strands 2 and 8 are adjacent. The structure has been determined to 1.6 A resolution in the laboratory of Hans Freeman in Sydney, Australia. (Adapted from J. Richardson.)...

Figure 4.14 Examples of different types of open twisted a/p structures. Both schematic and topological diagrams are given. In the topological diagrams, arrows denote strands of p sheet and rectangles denote a helices, (a) The FMN-binding redox protein flavodoxln. (b) The enzyme adenylate kinase, which catalyzes the reaction AMP +...

Figure 6.8 Schematic diagram of the enzyme DsbA which catalyzes disulfide bond formation and rearrangement. The enzyme is folded into two domains, one domain comprising five a helices (green) and a second domain which has a structure similar to the disulfide-containing redox protein thioredoxin (violet). The N-terminal extension (blue) is not present in thioredoxin. (Adapted from J.L. Martin et al.. Nature 365 464-468, 1993.)...

Although the reaction has the overall stoichiometry of a dehydration it is more complex than this and involves a mutual redox reaction between N and N. This is at once explicable in terms of the volt-equivalent diagram in Fig. 11.9 which also interprets why NO and N2 are formed simultaneously as byproducts. It is probable that the mechanism involves dissociation of NH4NO3 into NH3 and HNO3, followed by autoprotolysis of HNO3 to give N02, which is the key intermediate ... 

As may be seen from the potential-pH diagram " (Fig. 6.3) platinum is immune from attack at almost all pH levels. Only in very concentrated acid solutions at high redox potentials (i.e. under oxidising conditions) is there a zone of corrosion. This accounts for the solubility of platinum in aqua regia. Platinum is also prone to complex-ion formation, and this can lead... 

Electrolytic cell A cell in which the flow of electrical energy from an external source causes a redox reaction to occur, 481, 509q cell reactions, 498 diagram of, 486... 

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